Disrotatory versus conrotatory electrocyclic ring opening of Dewar benzene: the conrotatory pathway is preferred and does not involve trans-benzene

For the electrocyclic ring opening of Dewar benzene (2) into benzene (1), b oth a disrotatory and conrotatory pathway with distinct transition states, TS1 and TS2, respectively, were found at the CASSCF(10,10)/6-311G** level o f theory. The importance of the CASSCF(10,10) active space for the proper d escription of TS1 and TS2 was illustrated by similar calculations using a s maller active space, viz. CASSCF(2,2), CASSCF(4,4) and CASSCF(6,6). Althoug h TS2 represents a true saddle point, TS1 appears to be a higher order sadd le point at these levels of theory. Single-point multi-reference SDCI (MRCI ) calculations at the CASSCF(10,10)/6-311G** geometries and natural orbital s were performed at all stationary points to obtain more reliable total ene rgies. In contrast to common belief, TS2 lies below TS1 (by 6.62 kcal/mol), i.e. the conrotatory process is favored. Moreover, CASSCF(10,10)/6-311G** Intrinsic Reaction Coordinate (IRC) calculations show that upon conrotatory electrocyclic ring opening 2 does not give the extraordinarily strained tr aits-benzene (3), i.e. cis, cis, trans-cyclohexa-1,3,5-triene. The conversi on of 3 into 1 and vice versa is a distinct process on the C6H6 potential e nergy surface. (C) 1999 Elsevier Science B.V. All rights reserved.